Isotactic polypropylene is known to be one of the lightest major plastics. Yet, because of its high crystallinity, it is known to possess high tensile strength, stiffness and hardness. These characteristics allow finished materials made thereof to have good gloss and high resistance to marring. Further, its high melting point allows it to be subjected to elevated temperatures without loss of high tensile strength. However, because of the restriction of molecular motion characteristic of isotactic polypropylene brittle behavior takes place not far below room temperature and its poor low temperature impact strength limits its usefulness.
Different ways of improving the impact strength of the polypropylene at low temperatures without unacceptable adverse effect on its other properties, including its flexural rigidity and thermal resistance have been proposed.
U.S. Pat. No. 4,113,802, MATTEOLI et al., is directed to a process for producing polypropylene-based compositions with high impact strength by first polymerizing propylene in the presence of a catalyst such as TiCl.sub.3, and then adding ethylene or a mixture of ethylene and propylene and continuing the polymerization.
U.S. Pat. No. 4,128,606 FURUTACHI et al., is directed to preparation of impact-resistance polypropylene composition by first polymerizing propylene in the presence of a titanium-based catalyst and an organoaluminum compound; polymerizing propylene and ethylene in the presence of the foregoing reaction mix; and, in the presence of the reaction mix thus obtained, polymerizing either ethylene or both ethylene and propylene.
The usefulness of ethylene-propylene rubber ("EPR"), the general term for ethylene-alpha-olefin copolymer ("EPC")/ethylene-alpha-olefin-diene monomer ("EPDM") elastomeric polymers, for improving the impact strength of polypropylene ("PP") plastic compositions is known. The improvement may be generally accomplished through producing a simple physical mixture of PP with EPR.
For example, Japanese Patent No. 19934/67 is directed to producing shock-resistant polypropylene by adding an elastomer solution, which may be ethylene-propylene rubber, to polypropylene.
U.S. Pat. No. 4,087,485, HUFF, is directed to improving the impact strength of a polypropylene composition by incorporating therein minor amounts of polyethylene and ethylene-propylene copolymer.
As this literature exemplifies it is recognized that two or more polymers may be blended together to form a wide variety of random or structured morphologies to obtain products that potentially offer desirable combinations of characteristics. However, it may be difficult or impossible in practice to achieve many potential combinations through simple blending because of some inherent and fundamental problems. Frequently, the two polymers are thermodynamically immiscible, which precludes generating a truly homogeneous product. This may not be a problem per se since often it is desirable to have a two-phase structure. However, the situation at the interface between these two phases very often does lead to problems. The typical case is one of high interfacial tension and poor adhesion between the two phases. This interfacial tension contributes, along with high viscosities, to the inherent difficulty of imparting the desired degree of dispersion to random mixtures and to their subsequent lack of stability, giving rise to gross separation or stratification during later processing or use. Poor adhesion leads, in part, to the very weak and brittle mechanical behavior often observed in dispersed blends and may render some highly structured morphologies impossible.
The word "compatibility" has a technological usage in the polymer industry which refers to whether an immiscible polymer blend tends to form a stable dispersion, one less subject to problems of gross separation or stratification. A "compatibilizer" is a polymer that has the characteristics or properties permitting it to stabilize, or "compatibilize", a heterophase polymer blend.
It is generally known that the presence of certain polymeric species, usually block or graft copolymers suitably chosen, may serve as effective compatibilizers. This is believed to occur because of a preferential location of the compatibilizer at the interface of the phases in a blend. This preferential location most likely occurs as a result of entanglement of respective segments of the compatibilizer in the phases to which the segments are similar in chemical characteristics. This increases the adhesion between the phases and as a result of reduced surface energy between the phases better dispersion is permitted. The improved dispersion is observable directly by microscopic investigation of domain size of the dispersed phase. It has been suggested that ideally the compatibilizer component should be a block or graft with different segments that are chemically identical to those in the respective phases.
Certain polymer blends have previously been utilized with compatibilizers. U.S. Pat. No. 4,299,931 is directed to compatibilized polymer blends, wherein a blend of an olefin polymer and nitrile rubber is compatibilized by the addition of a block copolymer of the olefin polymer and the nitrile rubber.
U.S. Pat. No. 4,410,482 discloses the formation of a graft copolymer of nylon and polyethylene as part of a blend of nylon and polyethylene. The presence of the graft copolymer is said to have a dramatic effect on the properties of the blends (in this case, its permeability) which can be related to its function as a compatibilizer.
Likewise U.S. Pat. No. 4,264,747 discloses compatibilizing a blend of styrene acrylonitrile resins with styrene-ethylene-butylene-styrene (SEBS) block copolymer where the SEBS copolymer has been made compatible with the styrene acrylonitrile resin by forming a graft copolymer compatibilizer by grafting a polar monomer which may be the styrene acrylonitrile resin onto the SEBS backbone.
U.S. Pat. No. 3,739,042 discloses block copolymers prepared by first polymerizing an olefin or diolefin, or combinations thereof, for example, amorphous ethylene-propylene or ethylene-propylene-cyclopentadiene, in the presence of an appropriate anionic catalyst to form a first block, then polymerizing thereto at the still "living" catalytic site monomers which polymerize by a free radical mechanism, for example, acrylonitrile, styrene, etc. The block polymers of this invention are said to possess the unique ability to render dissimilar polymers compatible in one another. The linear block copolymers of this invention are further characterized by the fact that the anionically polymerized block obtained from alpha-olefins is normally substantially crystalline, i.e., it has a degree of crystallinity of at least 25%.
Despite the above knowledge in the art, a truly effective compatibilizer for blends of isotactic polypropylene ("i-PP") plastic compositions with EPR has not been available to the public or industry prior to the invention described herein and that described in co-pending companion case U.S. Ser. No. 264,485. The prior art block polymers all suffer to varying degrees the problem that where a single catalyst system is utilized the different segments will have characteristics arising from the catalyst system chosen and not necessarily the characteristics of the blend polymers with which they are utilized. Thus where i-PP is necessarily polymerized with catalyst systems yielding stereospecific polymers having the crystalline structure necessary for plastics, EPR is typically polymerized utilizing catalyst systems yielding substantially amorphous, random copolymers. Clearly the general goal of achieving chemical identity between compatibilizer segments and respective polymers in an EPR/i-PP blend is not met when a single stereo-specific catalyst system is used for both i-PP and random EPR segments.
Thus, the graft polymers of this invention, comprising EPR grafted with i-PP through functional linkages, are believed to be unknown prior to the disclosure herein.
Various methods have been developed for preparing the prior art block polymers having polymer segments differing from one another in composition.
European Patent No. 83-949-A discloses a thermoplastic block copolymer comprising one or more crystalline propylene blocks and one or more alkene - propylene blocks, in at least one of which diene units are present (constituting an EPDM block). The polymer is prepared by first polymerizing propylene, then polymerizing an EPDM and finally polymerizing propylene or ethylene. The process relates to the formation of substantially crystalline polypropylene and specifies the use of known high-stereospecific catalyst systems, exemplifying only TiCl.sub.3 -containing components. Dienes which are disclosed to be suitable in the preparation of the EPDM block include norbornadiene, dicyclopentadiene, tricyclopentadiene, 5-ethylidene-norbornene -2, 5-methylene -norbornene -2, 5 vinylnorbornene -1, and 5- (2-propenylnorbornene -2).
Japanese Patent 69/19,542 discloses a method for preparing propylene/ethylene block copolymers comprising carrying out polymerization using a stereospecific catalyst in a manner to achieve specific ratios of A and B blocks. The A block can be a propylene homopolymer and the B block can be an ethylene/propylene copolymer where the length of the B block can be regulated by the addition of a diene hydrocarbon. Suitable dienes included 1,5-cyclopentadiene. The catalyst exemplified comprises TiCl.sub.3. Japanese 69/20,751 contains a similar disclosure wherein propylene is polymerized alone, then propylene and 1,7 -octadiene and finally ethylene alone.
U.S. Pat. No. 3,454,675 discloses a method of preparing block polymers of mono -1- olefins using two reactors. The reactors are compartmented to prevent short circuiting of the catalyst in the first reactor which results in a short residence time for some of the catalyst in the first reactor. A first mono -1- olefin is polymerized in the first reactor, the polymer and its catalyst is transferred to the second reactor and the second mono -1- olefin is copolymerized therein. In one embodiment the reaction mixture of the first reactor is stripped of unreacted first mono -1- olefin before transferring it to the second reactor in order to achieve pure block polymer. In another embodiment the unreacted monomer is transferred with polymer and catalyst to the second reactor. The result is a mixed block copolymer that can comprise a polypropylene segment and an ethylene-propylene copolymer segment. Catalyst systems are based on transition metal halides of titanium, zirconium, hafnium or germanium, TiCl.sub.3 is preferred.
U.S. Pat. No. 3,268,624 discloses a method for preparing a two segment block copolymer of ethylene and propylene which comprises first polymerizing a feed comprising propylene and propylene with a small amount of ethylene using a catalyst comprising titanium trichloride, an alkyaluminum dihalide, and an alkoxy silane. After the polymerization has proceeded for the desired length of time the first (propylene) feed is discontinued and a second feed of ethylene or ethylene with a small amount of propylene is fed to the reactor.
U.S. Pat. No. 3,301,921 discloses a composition of matter comprising a highly isotactic polypropylene polymer chain, uninterrupted by ethylene, having attached thereto, at one end, an ethylene-propylene copolymer. The process for forming the composition of the invention utilizes catalyst and operation conditions selected to produce stereospecific polymers. The ethylene content of the block polymer is about 1 to 20 wt. % while the ethylene content of the ethylene-propylene segment is about 10 to 90 wt. %. The product is said to have improved impact resistance over polypropylene alone. The propylene polymerization is carried out to about 90 to 95% of the desired propylene conversion. Either the polypropylene or the ethylene-propylene copolymer can be produced first, in both cases the first polymerized monomer(s) are contacted with a stereospecific catalyst with subsequent addition of the second monomer(s) to the reaction mix. The catalyst used is TiCl.sub.3 with aluminum alkyl or aluminum alkyl halides. U.S. Pat. No. 3,318,976 discloses and claims the process for preparing the product claimed in the '921 patent. Both patents are continuation-in-part applications based on the same earlier filed application (Ser. No. 77,776 filed Dec. 22, 1960).
Ziegler-Natta catalysis is capable of producing highly isotactic therefore highly crystalline polymers and in addition can be used to polymerize a wide range of monomers including ethylene and propylene. Additionally Ziegler-Natta catalysis can be utilized to produce random, elastomeric copolymers from the same readily available monomers depending upon the choice of catalyst system. However, this method of catalysis results in polymerization of very short duration making sequential polymerization of crystalline and random polymer segments difficult or impossible. A method was sought therefore that could utilize the benefits of Ziegler-Natta polymerization to produce a polymer composition having both crystalline PP segments and highly random, substantially amorphous EPR segments to serve as both a compatibilizer for PP/EPR blends and a PP impact strength improver.